Protein kinases have gained acceptance as therapeutic targets and have become a major focus of drug development efforts in oncology, with hundreds of inhibitors either in the pipeline or already in the clinic. Protein phosphatases, on the other hand, have been largely ignored for drug development because of their reputed lack of substrate specificity and the toxicity associated with natural products discovered as potent active site inhibitors. In contrast to the narrow substrate specificity of protein kinases, PP2A interacts with multiple substrates, and therefore its activation is, in effect, a combination therapy that coordinately inhibits multiple signaling pathways, including oncogenic signaling pathways. Protein phosphatase 2A (PP2A), composed of ABC subunits, dephosphorylates key oncogenic signaling proteins to function as a tumor suppressor.
Protein phosphatase 2A is one of the four major serine threonine phosphatases and is implicated in the negative control of cell growth and division. Protein phosphatase 2A holoenzymes are heterotrimeric proteins composed of a structural subunit A, a catalytic subunit C, and a regulatory subunit B. The PP2A heterotrimeric protein phosphatase is a ubiquitous and conserved phosphatase with broad substrate specificity and diverse cellular functions. Among the targets of PP2A are proteins of various signaling cascades, such as Raf, MEK, AKT, ERK, and FOXO.
PP2A interacts directly with FOXO1 and dephosphorylates FOXO1. Inhibition of PP2A phosphatases rescues FOXO1-mediated cell death by regulating the level of the pro-apoptotic protein BIM. In addition, PP2A directly regulates FOXO3a subcellular localization and transcriptional activation. Without wishing to be held to any particular theory, it may be that the compounds described herein promote apoptosis by acting on FOXO transcription factors via activation of PP2A.
The FOXO (Forkhead transcription factors, Class O) proteins are a group of transcription factors involved in control of a variety of physiological, metabolic and developmental pathways. They are downstream effectors in a number of signaling pathways including insulin and growth factor signaling; they are also regulated by oxidative stress and nutrient deprivation. Cellular processes affected by FOXO activity include cell cycle control, differentiation, proliferation and apoptosis. Disregulation of FOXO mediated processes has been implicated in a number of pathologies including tumorigenesis, inflammation, diabetes and neurodegenerative conditions amongst others. Activity of FOXO transcription factors are controlled in part by their sub-cellular localization, in particular their localization to the nucleus from the cytosol, and their subsequent transcriptional activation.
Four FOXO proteins designated FOXO1, FOXO3a, FOXO4 and FOXO6 are present in human cells and their activity is controlled by a variety of mechanisms including stability (proteolytic cleavage), sub-cellular localization and transcriptional activation. Activity of the first three members of the family is controlled by cytosolic-nuclear translocation.
FOXO1 regulates expression of a number of genes that play critical roles in cell cycle and apoptosis. A pivotal regulatory mechanism of FOXO is reversible phosphorylation, catalyzed by kinases and phosphatases. Phosphorylation of FOXO1 is associated with 14-3-3 binding and cytosolic localization, whereas dephosphorylated FOXO1 translocates to the nucleus and is transcriptionally active.